It is sometimes necessary or desirable for a physician to determine the concentration of certain gases, e.g., oxygen and carbon dioxide, in blood. This can be accomplished utilizing an optical sensor which contains an optical indicator responsive to the component or analyte of interest. The optical sensor is exposed to the blood, and excitation light is provided to the sensor so that the optical indicator can provide an optical signal indicative of a characteristic of the analyte of interest. For example, the optical indicator may fluoresce and provide a fluorescent optical signal or it may function on the principles of light absorbance.
The use of optical fibers has been suggested as part of such sensor systems. The optical indicator is placed at the end of an optical fiber which is placed in contact with the medium to be analyzed. This approach has many advantages, particularly when it is desired to determine a concentration of analyte in a medium inside a patient's body. The optical fiber/indicator combination can be made sufficiently small in size to easily enter and remain in the cardiovascular system of the patient.
Optical fluorescence CO.sub.2 sensors commonly utilize an indirect method of sensing based on the hydration of CO.sub.2 to form carbonic acid within an optionally buffered aqueous compartment containing a pH sensitive dye. The aqueous compartment is encapsulated in a barrier material which is impermeable to hydrogen ions but permeable to CO.sub.2. An optically interrogated pH change in the internal aqueous compartment can then be related to the partial pressure of CO.sub.2 in the monitored sample. Ionic isolation of the internal aqueous phase may be achieved by directly dispersing aqueous droplets throughout the isolating matrix. Alternatively, the aqueous phase may be sorbed into porous particles which are then dispersed throughout the isolating matrix. The isolation matrix or "barrier" is typically a crosslinked silicone polymer.
Unfortunately, prior attempts to provide stable and reproducible emulsions of an aqueous phase dispersed in a polymeric precursor have yielded poor results. In some cases, the emulsions exhibited unexplained lot-to-lot variability that frustrate attempts to perform quantitative experiments correlating sensor performance to the particular sensor formulation. Variability within lots has also been observed. This variability frustrates attempts to uniformly produce sensors, e.g., by coating a sheet of sensor precursor and converting the sheet into individual sensor elements. In other cases, the emulsoids formed from the emulsions are adversely affected by heat (e.g., during autoclaving) and the sensor's performance is thereby compromised. Also unfortunately, prior attempts to make sensors that respond to CO.sub.2 in the "dry" state, i.e., not in contact with liquid water, have yielded poor results. Traditional two-phase sensors dehydrate when stored in ambient conditions and lose intensity. Even when intensity is maintained in the dry state, the sensor may not respond to CO.sub.2.
It would be desirable to provide a stable and reproducible sensor which has a fast response time and which is easily manufactured. It would also be desirable to provide a CO.sub.2 sensor that provides a stable and effective signal that does not require that it be held in a condition of equilibrium with liquid water or saturated water vapor.